Image sensors are semiconductor devices for converting an optical image into an electric signal. Charge coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors are the most representative devices of the image sensors.
In a CCD, metal oxide silicon (MOS) capacitors are arranged in close proximity to each other, and charge carriers are stored in the MOS capacitors and transferred between the MOS capacitors.
A CMOS image sensor may include a plurality of MOS transistors which correspond to a plurality of pixels. The MOS transistors may be formed based on CMOS technology using a control circuit and a signal processing circuit which may be used as peripheral circuits. The CMOS image sensor may serve as a switching device for sequentially detecting outputs using the MOS transistors.
A photodiode and a plurality of MOS transistors may be included in each pixel of the CMOS image sensor. The photodiode may serve to generate an optical charge in response to incident light.
The plurality of MOS transistors may include a transfer transistor, a reset transistor, an access transistor, and a select transistor. The transfer transistor may be connected to the photodiode to transfer optical charge from the photodiode to a floating diffusion region. The reset transistor may be used to set the electric potential of the floating diffusion region to a desired value and reset the floating diffusion region by discharging the floating diffusion region. A voltage of the floating diffusion region may be applied to a gate of the access transistor such that the access transistor is used as a source follower buffer amplifier. The select transistor may be used for addressing by using a switching function.
The transfer transistor may include a gate, a channel through which charge carriers moves, and a drain (hereinafter, referred to as a floating diffusion region) used as a floating diffusion region. The gate may be formed between device isolation pattern lines that define an active region and a field region on and/or over a semiconductor substrate. The gate may include a gate insulation layer and a polysilicon layer.
The channel may be used to control a threshold voltage and transfer an optical charge from the photodiode to the floating diffusion region. The channel may be formed in the semiconductor substrate in correspondence with the gate. The channel may be formed by implanting ions into the semiconductor substrate.
The photodiode and the floating diffusion region may be formed at both sides of the channel. The floating diffusion region stores a charge transferred from the photodiode through the channel.
The operation of the transfer transistor will now be briefly described. First, the photodiode generates an optical charge in response to incident light, and then, the gate of the transfer transistor is turned on. As a result, a threshold voltage controlled by the channel is reduced, and thus, the optical charge can be transferred from the photodiode to the floating diffusion region through the channel.
However, in such a CMOS image sensor, the characteristics of the transfer transistor may be deteriorated due to structural defects. In this case, when the gate is turned off, a leakage current can be generated. Furthermore, when the gate is turned on, the optical charge cannot be completely transferred from the photodiode to the floating diffusion region, thereby resulting in a decrease in saturation level and image quality.
This problem may be due to the thickness of a gate insulation layer of the transfer transistor. For example, the thickness of the gate insulation can be increased to increase a threshold voltage so as to prevent the optical charge generated in the photodiode from leaking through the channel. However, in this case, when the gate is turned on, the optical charge cannot be completely transferred from the photodiode to the floating diffusion region as described above.
On the other hand, the thickness of the gate insulation layer can be decreased to decrease the threshold voltage so as to completely transfer the optical charge from the photodiode to the floating diffusion region when the gate is turned on. However, in this case, the optical charge can leak from the photodiode through the channel.